1. Field of the Invention
The invention relate to a drive circuit for the brushless motor.
2. Description of the Prior Art
The rotation driving control of a brushless motor can be classified roughly into two functions. One of which is a commutation control for controlling the timing of respective phase current which flows through respective armature winding. Another is a speed control which maintains the rotation speed constant.
In the commutation control, a rotor location signal is necessary which indicates the relative position of the armature winding and the rotor. On the other hand, in the speed control, a speed signal is necessary for indicating rotation speed of the rotor.
For the commutation change control of conventional typical brushless motor, a rotor location detecting element such as Hall device is used. But, since the rotor location detecting element is no so cheap and needs further many electric wiring, there is some demerit which is complicated and causes an increase of cost.
Further, by using the rotor location detecting element, the thinness and small sizing of the motor is restricted. Since an output of the rotor location detecting element changes by temperature and humidity, there remain some problems from the view point of reliability.
To avoid this demerit, some driving systems for the brushless motor are proposed for detecting a rotor location using a counter electromotive voltage signal induced in the armature winding. The typical driving system for detecting the rotor location from the counter electromotive voltage signal is disclosed in the laid-open Japanese Patent Publication No. 58-25038. In the above publication, two systems are disclosed, one of which is a system for detecting a location by comparing both terminal voltages of respective armature winding and the other system by comparing the terminal voltage and the neutral voltage of the armature winding.
FIG. 120 shows a general construction of the system for detecting the location by comparing the terminal voltage and the neutral voltage of the armature winding. In FIG. 120, 12, 13 and 14 are the armature windings of a star connection brushless motor, 11 is a bridge circuit which switches the current flowing in the armature windings 12, 13 and 14. A comparator 500 compares the terminal voltages U, V, W, each of which is not neutral terminal of the armature winding, with a neutral voltage M, respectively, to detect the rotor location. A commutation control means 501 controls the bridge circuit 11 according to the rotor location which is detected by the comparator 500 and makes the rotor rotate by supplying a current into the predetermined phase of the armature winding.
Further, the laid-open Japanese Patent Publication No. 51-100216 discloses a method for compensating a phase delay generated when comparing the terminal voltages, where ohmic drop of the armature winding resistor is assumed to be constant and the phase delay is compensated using a shunt resister.
By the way, in the driving system for detecting a rotor location using the counter electromotive voltage induced in the armature winding, the rotor location can not be detected if the counter electromotive voltage is less than a predetermined value by the reason of low rotation speed of the rotor. On this occasion, since the rotor location can not be obtained at starting, it is necessary to forcefully apply a rotating magnetic field to the rotor from outside.
But, in case applying forcefully the rotating magnetic field to the rotor from outside, the relative position of the rotor and the armature winding do not always stop to a location where the rotor can rotate to a normal direction, but some torque may be generated toward a counter rotation direction at the beginning of starting at some location of the rotor, which involves serious problems in which a normal starting is prevented.
To solve this problems, some improvements are proposed, for example, in the laid-open Japanese Patent Publications No. 57-173385 and No. 2-237490. In the laid-open Japanese Patent Publication No. 57-173385, it is disclosed a starting system, which starts to rotate a rotor after having fixed the rotor to a predetermined position at starting, comprises a means for supplying a current to a predetermined phase of the armature winding at starting for a predetermined period and a means for forcefully applying a rotating magnetic field to an armature winding from the outside.
The system of the laid-open Japanese Patent Publication No. 57173385 explains using FIG. 121(a) that a fixed timer circuit 512, the armature windings 12 (U phase), 13 (V phase) and 14 (W phase) of a three phase DC brushless motor are connected to a power supply 510 through a power switch 511. The armature windings 12, 13 and 14 is connected to collector of transistor 515, 516 and 517, respectively and an emitter of this transistor is connected to earth, which comprises a driving circuit 518.
A switching timer circuit 513 and a rotating magnetic field generating circuit 514 are connected to the fixed timer circuit 512, and a switching circuit 519 is connected to this switching timer circuit 513. The switching circuit 519 switches currents outputted from the rotating magnetic field generating circuit 514 which drives the driving circuit 518 and driving currents detected at the armature windings 12, 13 and 14 according to the electromotive voltage to supply to the driving circuit 518. The output terminals of this switching circuit 519 are connected to respective phases of the bases of the transistors 515, 516 and 517. An electromotive voltage detecting circuit 520 is connected to inputs of the switching circuit 519. The armature windings 12, 13 and 14 are connected to the input terminals of this electromotive voltage detecting circuit 520. Respective three phase output signals from the rotating magnetic field generating circuit 514 is inputted to the input terminals of this switching circuit 519.
Since the device is constituted in this way, when the power switch 511 is turned on, one of the armature windings 12, 13 and 14 is connected to the power supply 510 and the fixed timer circuit 512 operates simultaneously. The fixed timer circuit 512 outputs a control signal 521 to the rotating magnetic field generating circuit 514 for a predetermined period, the control signal 521 excites one of the armature windings 12, 13 and 14, specifically, for example, a W phase shown in FIG. 121(b). The fixed timer circuit 512 also outputs a control signal 521 to the switching timer circuit 513. After a predetermined period when the control signal 521 is outputted from the fixed timer circuit 512, the fixed timer circuit 512 turns off. When the fixed timer circuit 512 turns off, the rotating magnetic field generating circuit 514 outputs driving signals 523, 524 and 525 as shown in FIG. 121(b) to the bases of the transistors 515, 516 and 517 through the switching circuit 519, where the driving signals 523, 524 and 525 excite the armature windings 12, 13 and 14, respectively. This switching circuit 519 is switched to a state of solid line shown in FIG. 121(a) according to a switching indication signal 522 shown to FIG. 121(b), and this state is maintained until the switching indication signal 522 outputted from the switching timer circuit 513 becomes low (L) state.
The driving signals outputted from the rotating magnetic field generating circuit 514 applies driving signals sequentially to the bases of the transistors 515, 516 and 517 in the driving circuit 518. The sequential currents are supplied into the armature windings 12, 13 and 14 from the power supply 510. Therefore, the rotating magnetic field is generated in the armature windings 12, 13 and 14, which causes the rotor to begin rotating.
The motor is started in this way. When a predetermined period is passed, a switching signal 522 which is outputted from the switching timer circuit 513 becomes L state and the switching circuit 519 switches its contacts the driving signals outputted from the voltage detecting circuit 520 is applied to the driving circuit 518, which drives the transistors 515, 516 and 517 sequentially and maintains rotation of the three phase brushless motor.
The prior art of the laid-open Japanese patent publication No. 2-237490 is explained below. This brushless motor has an electromagnetic transducer for detecting a rotation position of the rotor. This brushless motor selects one predetermined current switching pattern corresponding to a rotor stopping location from the predetermined current patterns set for starting according to the rotating location detected by the electromagnetic transducer at starting, then switches the driving current to be supplied to the stator armature winding by this selected current switching pattern to generate a rotating magnetic field and to cause the rotor to start. When the electromotive voltage generated in the stator armature winding reaches a predetermined value which is necessary to detect the rotation position of the rotor, the rotation position of the rotor is determined from the detected electromotive voltage. By the detected output, the driving current for driving the armature winding is switched to generate the rotating magnetic field and to cause the rotor to rotate.
On the other hand, in a speed control of the brushless motor, it is generally used a system for maintaining the rotating speed constant by controlling the current quantity flowing in the armature winding.
FIG. 122 shows a block diagram of a speed control system of the driving circuit for conventional brushless motor. In FIG. 122, 530 is a speed detecting circuit which detects actual rotation speed of the rotor and outputs a speed signal, 531 is a speed difference detecting circuit which outputs a speed difference signal having a pulse width corresponding to the speed difference by counting the speed signal period using reference clock. A speed difference compensation filter 532 outputs a current indication value to a current supplying circuit 533 so that a speed difference becomes zero according to the speed difference signal. The current supplying circuit 533 regulates a current quantity supplied to the armature winding of the brushless motor 534 according to the current indication value. In the driving circuit of such conventional brushless motor, the speed difference compensation filter is constituted of an analog filter in which a PI filter 460 and a first order delay filter 464 are connected serially as shown in FIG. 123.
In the driving circuit of the conventional brushless motor, the speed signal period is counted by the reference clock and therefore the reference clock frequency inputted into the speed difference detecting circuit is switched in proportion to the indicated rotation speed when the indicated rotation speed to the motor is switched.
Regarding the system which detects the speed signal for controlling the rotation speed of the brushless motor, there are a system which is popularly called a FG system which exclusively use a frequency generator for speed detection and a system which detects speed according to a feature that a magnitude of the counter electromotive voltage signal induced in the armature winding is in proportion to the rotation speed.
In the conventional driving system in which the location detection is carried out by comparing the terminal voltages each other, since one of the terminal is a current supplying phase and another is a non-current supplying phase, when the current in the current supplying phase increases at load state, the ohmic drop in the armature winding of current supplying phase becomes large, and therefore the rotor location signal of the current supplying phase delays.
This delay causes a commutation timing to delay, then the torque decreases, which causes a rotation speed to decrease. When the rotation speed decreases, the current increases in order to rise the rotation speed, and the ohmic drop further becomes large, that is, there occurs a vicious circle. In the worst case, the motor can not generate a torque which is larger than the load, and then the motor will fall into stopping state.
On the other hand, in the conventional driving system in which the location detection is carried out by comparing the neutral voltage with the terminal voltages, although the phase delay of the location detecting signal can be solved, it is necessary to draw out the neutral leads from inside of the brushless motor.
The rotor location signal obtained by comparing the neutral voltage with the terminal voltages is shifted around 30 degrees in electrical angle against the actually needed rotor location signal, then the compensation must be carried out. The integration filter is usually used for phase compensation, therefore, when the motor is derived in a variable speed, the integration filter constant must be changed. Accordingly, if the constant is fixed, the commutation operation becomes unstable at transient state.
Further, in the above both driving system, by the influence of spike shaped noise generated on the terminal voltage waveform accompanied with the switching of the driving transistor, the rotor location signal becomes incorrect.
In the detection system for detecting a speed by a private frequency generator, it is necessary to provide a frequency generator having a high machining accuracy. Therefore, it is necessary to provide a wide space for installing the private detecting apparatus and then there is a cost disadvantage.
In the detecting system for detecting a speed by an amplitude of the counter electromotive voltage, since the voltage generated by the current flowing in the armature winding is superposed to the counter electromotive voltage, it is difficult to detect only the amplitude of the counter electromotive voltage, and also the magnitude is followed by a change of environment.
In the conventional brushless motor, it needs a predetermined time to start after a starting switch is turned on.
In the conventional starting system, when the starting is failed, it is necessary to restart and so it takes a lot of time for starting the motor.
In the conventional speed difference compensation filter in the driving circuit, when disturbance at low pass area is large, the disturbance can not be satisfactorily compressed. Therefore, the specification of the rotation accuracy can not be well satisfied.
In the conventional driving circuit for the brushless motor, when the indicated rotation speed to the motor has changed, it is necessary to change the reference clock inputted into the speed difference detector in proportion to the indicated rotation speed.
In the conventional driving circuit for the brushless motor, a commutation timing is not synchronized with a timing for increasing or decreasing the armature winding current. Therefore, it is difficult to add or subtract a compensation value which is determined by the resistance value of the armature winding and the current flowing in the current supplying phase to/from the winding voltage during the actual driving period.
In the conventional driving circuit for the brushless motor, the transistors are switched by the rectangular shaped wave, there occurs noise at commutation.
It is an object of the present invention to provide a driving circuit for the brushless motor having no phase delay in the rotor location signal even at load state, having a stable rotation irrespective of steady state or transient state, and a simple lead connection.
Further, it is an object of the present invention to provide a driving circuit for the brushless motor for correctly detecting a rotor location signal even if spike shaped voltage fluctuation is generated on the terminal voltage waveform accompanied with switching of driving transistor.
Further, it is an object of the present invention to provide a driving circuit for the brushless motor for obtaining a speed signal without providing a private speed detecting circuit.
Further, it is an object of the present invention to provide a driving circuit for the brushless motor for being started in a short time without influenced by the load condition.
Further, it is an object of the present invention to provide a driving circuit for the brushless motor for providing a high accurate rotation mode with fully compressed disturbance even if the disturbance at low pass area is large.
Further, it is an object of the present invention to provide a driving circuit for the brushless motor for changing the rotation stably by switching a target rotation speed and a gain of the speed difference compensation filter.
Further, it is an object of the present invention to provide a driving circuit for the brushless motor for synchronizing the commutation timing with a timing for increasing or decreasing the armature winding current.
Further, it is an object of the present invention to provide a driving circuit for the brushless motor having less noise at commutation operation.